The present invention generally relates to interferometer devices and methods and their sensing application.
Distributed sensing technology replaces complex interrogation of thousands of sensors by a single waveguide (i.e. optical fiber cable or coaxial cable) that consists of many sensors in series. It makes almost continuous measurement over the full length of the cable instead of discrete measurement. Real time interrogation can also be implemented. As such, a truly continuous monitoring at all points along a cable can be obtained at all times through the technology of distributed sensing.
Several techniques have been suggested to realize distributed sensing. One technique is called frequency division multiplexing (FDM). The sensors on one cable have slightly different interrogated frequencies. A frequency-tunable source can be tuned to the frequency belonging to one particular sensor. The influences of the desired parameters such as strain or temperature could shift its interrogated frequency. The measurements can be further correlated by tracking the shift in frequency. Fiber Bragg gratings (FBGs) are typically used in FDM. It can have a high measurement resolution because of phase detection for each sensor. However, the maximum number of sensors along one cable in any case is typically between 10 and 100, limited by the tuning range or bandwidth of the light source and the required frequency interval per sensor.
Another technique, called time division multiplexing (TDM), uses cascaded weak reflectors along a cable. Each sensor/reflector is interrogated with short pulses in time domain. The reflections from different reflectors are then distinguished via the differences in time-of-arrival. This method could potentially multiplex more than 100 sensors but has a relatively low measurement accuracy.